1. Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a substrate that is used for creating a Ball Grid Array package.
2. Description of the Prior Art
The semiconductor industry has since its inception achieved improvements in the performance of semiconductor devices by device miniaturization and by increasing the device packaging density.
One of the original approaches that has been used to create surface mounted, high pin count integrated circuit packages has been the use of the Quad Flat Pack (QFP) with various pin configurations. For the QFP, closely spaced leads along the four edges of the flat package are used for making electrical connections from where the electrical connections are distributed to the surrounding circuitry. The input/output (I/O) connections that can be made to the QFP are therefore confined to the edges that can be made to the QFP even in applications where the pin to pin spacing is small. The QFP has found to be cost-effective for semiconductor devices where the device I/O pin count does not exceed 200. To circumvent this limitation, a new package, a Ball Grid Array (BGA) package has been introduced. For the BGA package, the electrical contact points are distributed over the entire bottom surface of the package, eliminating the restriction of having I/O connects only around the periphery of the package. More contact points with greater spacing between the contact points can therefore be allocated across the BGA package than was the case with the QFP. The contact points that are used for the BGA package are typically solder balls that have the added advantage of facilitating reflow soldering of the package onto a printed circuit board.
Prior Art substrate packaging uses ceramic and plastic BGA packaging. Ceramic substrate packaging is expensive and has proven to limit the performance of the overall package. Recent years have seen the emergence of plastic BGA packaging; this packaging has become the main stream design and is frequently used in high volume BGA package fabrication. The substrate of Plastic BGA (PBGA) package performs satisfactorily when used for low-density flip-chip IC""s. If the number of pins emanating from the IC is high, that is in excess of 350 pins, or if the number of pins coming from the IC is less than 350 but the required overall package size is small, or if the chip power dissipation is high (in excess of 4 Watts per chip), the substrate structure becomes complicated and expensive.
The invention addresses placing of a heatsink that is used in PBGA packages in either a die-up and or a die-down mold chase.
U.S. Pat. No. 5,872,395 (Fujimoto) shows a heat spreader using a mold compound and a mold cavity.
U.S. Pat. No. 5,641,987 (Lee) shows another similar heat spreader we design.
U.S. Pat. No. 5,977,626 (Wang et al.) U.S. Pat. No. 6,201,301 (Hoang) and U.S. Pat. No. 5,834,839 (Mertol) show related heat spreaders and methods.
A principle objective of the invention is to provide a method of mounting a heat shield over a semiconductor substrate such that the heat shield is positioned precisely, preventing problems of heat shield shifting or tilting.
Another principle objective of the invention is to provide a method of mounting a heat shield over a semiconductor substrate such that the heat shield is firmly held in place, preventing problems of heat shield shifting or tilting.
Another objective of the invention is to apply a heat sink over the surface of a substrate without the need for adhesive material.
Another objective of the invention is to provide a method for improved heat dissipation from the heat sink into the underlying substrate.
Yet another objective of the invention is to provide effective grounding connection between the heat sink and the surface of an underlying substrate.
A still further objective of the invention is to remove the antenna effect that typically caused by the heat sink of a PBGA package.
A still further objective of the invention is to provide good electromagnetic shielding of the semiconductor device that is mounted in a PBGA package.
In accordance with the objectives of the invention a new method is provided to position and secure a heat sink over the surface of a semiconductor device mounting support, the latter typically being referred to as a semiconductor substrate. A plurality of recesses is created in the surface of the substrate over which the heat sink is to be mounted. The heat sink is (conventionally and not part of the invention) provided with dimples that form the interface between the heat sink and the underlying substrate. The dimples of the heat sink are aligned with and inserted into the recesses that have been created by the invention in the underlying substrate for this purpose, firmly securing the heat sink in position with respect to the substrate.